Reciprocating motion in machinery is usually generated by means of a simple crank mechanism where pure sinusoidal motion is not required; or by means of a scotch yoke mechanism (crank and slide) where pure sinusoidal motion is required. In either case adjustment of the output motion amplitude requires stopping the machine to change the crank arm length.
In many machine applications it would be desirable to have a cyclic reciprocating motion which is infinitely adjustable in amplitude from zero to maximum without stopping or disconnecting the input or output.